A disc drive data storage system includes a data storage disc providing a disc surface that has concentric data tracks, a data head that can read/write to the disc surface, and an actuator which supports the data head above the disc surface. Movement of the actuator causes movement of the data head.
In disc drives, an access can be defined as the movement of the data head relative to the disc surface from the end of one read/write on the disc surface to the beginning of the next read/write on the disc surface, but not including the actual reading/writing. The access time is made up of two time components: seek time and latency time. The seek time component is the time taken to travel from the track on which one read/write completes to the track on which the next read/write begins. The second time component, the rotational latency time, includes the time the disc drive spends waiting for the appropriate data to rotate under the head after the head has arrived at the track on which the next read/write begins. The rotational latency time also includes the time the disc drive spends at a track after data has been read or written to the desired location. The rotational latency time can be a significant part of the access time for all but the longest seeks. It is the dominant component of the access time for relatively short seeks. In some disc drive data storage systems, commands sent by a host computer are executed in an optimum order to minimize both radial movement of the data head and also rotational latency.
In spite of the advantages gained by optimizing the execution of commands, lack of control over the number and size of commands and the time between commands arriving from the host computer imposes limitations in the optimization techniques. Thus, significant rotational latency still exists in current systems.
Access operations are typically a combination of rapid movement of the actuator that moves the data head to the desired track followed by a wait period for the appropriate data to come under the head. Rapid movement of the actuator causes problems in the disc drive data storage systems such as increased vibration, an increase of emitted acoustics, high power consumption, and generally a higher stress factor to the overall mechanics.
When performing read seeks, pre-fetch or post-fetch data can be obtained from the target data track during the latency period before the desired data rotates beneath the head and after the desired data is read. Accessing the pre-fetch and post-fetch data improves the drive performance, but results in increased acoustic noise due to the relatively high velocity of the seek required to arrive at the target track soon enough to read the pre-fetch data. Conventional just-in-time seeking, in contrast, calls for seek operations to be performed at the slowest possible velocity which allows the head to arrive at the data to be read by the time that location rotates beneath the head. This reduces the level of acoustic noise, but ignores the potential efficiency gains attainable by reading pre-fetch or post-fetch data in the same fetch operation as that of the target data.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.